Thermal and Hydrologic Properties of Old Oceanic Crust in Hole 765D, Argo Abyssal Plain, Indian Ocean

Abstract

With a few exceptions, our knowledge of heat flow within the oceanic lithosphere comes from seafloor measurements using techniques that include sediment probes and coring. Borehole temperature-logging technology has been remarkably successful in continental boreholes, and has only recently been exploited by the ODP community. Borehole temperature measurements are valuable because the data gathered allow us to investigate how heat transfer (conductive and/or convective) varies with depth. For this reason, fine-scale, high-resolution temperature profiles were measured in Hole 765D to document the thermal and hydrologic properties of 140-Ma-old oceanic crust in the Indian Ocean near the northwestern shelf of Australia. Three complete temperature logging runs were conducted employing the new temperature logging tool (TLT), which is now standard logging equipment aboard JOIDES Resolution. The results indicate an average heat flow for the sediment section (0-932 mbsf) of 52.8 ± 5.7 mW/m 2 . This heat flow is slightly higher (~9 mW/m 2 ) than both regional heat-flow measurements and the heat flow predicted by a conductive cooling model. Temperature profiles for the basalt section (932-1195 mbsf) are each associated with temperature inversions and intervals (10-50 m) of near-constant temperatures. The computed equilibrium temperature profile is thermally depressed in excess of a few degrees Celsius compared with an expected profile assuming a conductive and steady-state heat-flow regime. The major source of this thermal perturbation appears to be formation fluid that enters the borehole near the sediment/basalt interface and eventually flows downhole and into the basalt formation, presumably through open fractures. The flow is generally restricted to the upper 150 m of the basalts. Based on these thermal and hydrologic observations, the shallow section of the oceanic crust at this site is believed to be permeable and slightly underpressured (fluid pressure less than hydrostatic).

Thermal conductivity prediction from petrophysical data: a case study

“…The values for heat flow using core measurements vary from 50.74mW/m 2 (complete section) to 65.66 roW/ m 2 (basalt only), whereas using log-derived measurements the range is from 48.2 mW/m 2 to 62 mW/m 2. These values approximate the range reported by Castillo (1991) using the interval method. The surface temperature estimates range from 0.63 to 7.9°C compared to a measured value of 2.5°C.…”

“…Using the temperature profile from Castillo (1991), values of kp range from 0.56 at the seabed to 0.62 at ll60mbsf.…”

“…8 the inverted conductivity predictions from wireline logs, using the algorithm described above, are multiplied by a constant and plotted as a function of depth alongside the nine-term smoothed temperature gradient derived from the high-resolution temperature survey (Castillo 1991). Using a constant of 52mW/m z gives reasonable agreement in the sediment column.…”

“…Using the thermal interval method (Powell et al 1988), Castillo (1991 derived a conductive heat flow value for the sediment of between 42+_4mW/m 2 to 60_+8mW/m 2 with a weighted average value of 51.02 _+ 0.12 mW/m 2. Using a method proposed by Bullard (1939), a figure of 51.21 mW/m 2 was obtained.…”

“…Using a method proposed by Bullard (1939), a figure of 51.21 mW/m 2 was obtained. Considering several techniques, Castillo (1991) concluded that the mean present day heat flow for Hole 765D is 52.8 +_ 5.7mW/m 2. In the Bullard method a conductive steady-state heat flow model is assumed.…”